here's the FAQ regarding the fee from Sprint's community forums
Plans: What's there to know about the $10 Smartphone Premium Data Add-on?
ok so sprint says that now after 01/30/11, new activations of smartphones whether it be the Evo, Epic, Shift or any other smartphones they have will now require the $10 premium data-add on. ok...so going with them classifying these 4g phones as smartphones instead of UFO's (Unidentified Fondled Objects) with this new change in the way they charge the $10, you would think it would change when it comes to SERO customers to, right? wrong!!
if you're on SERO and decide to get a smartphone...say like the Evo, common sense would say you'll only have to upgrade to the SERO premium now. remember, the $10 premium data fee now applies all smartphones (3g and 4g alike). so that would mean SERO premium is all you'll need because that's the SERO plan if you wanted a smartphone. WRONG!! based on that FAQ, you'll still have to upgrade to SERO premium and then pay the extra $10/mo for having the 4g phone making it $50/mo. so that means that for SERO, it's still a $10/mo cool fee.